In integrated circuit (IC) layout, wide wires (for example, wider than ten microns) are designed for carrying large electrical current. For example, power buses are usually wide metal wires formed from copper or aluminum to obtain low resistance. However, in the course of manufacturing, a chemical mechanical polishing (CMP) process to planarize the metal layers and other portions of the circuit often produce concave “dishing” in the metal surface, where dishing is more severe where the metal wire is wider. That is, when polished, the center part of the surface of the embedded metal wire forms a “dish-like” profile where the thickness of the metal wiring is reduced at the center. Furthermore, CMP heavily depends on the metal width and density. Metal dishing is more serious for larger line widths and where portions of an integrated circuit have a high area fraction containing metallization.
To remedy this problem, shapes (“cutting forms”) in the form of polygons (e.g., rectangular slots) are placed into metal wires during design layout to indicate where to cut holes within the metal wire pattern. The holes lead to a reduction of the effective metal width and area density in any local region, breaking up the larger expanse of the metal wire width, and reducing dishing effects, thus limiting the thinning of the metal wire. When adding the cutting forms in the shape of elongated slots, it is highly desirable to place the long dimension of the cutting forms parallel to the direction of electrical current flow. This is done to facilitate the current flow and therefore reduce the effect of electromigration, a cause of increased path resistance, conductivity degradation and failure.
Conventionally, cutting forms are added into a layout manually. This can be a time consuming process. Another method may include inserting the rectangles into wide metals using a conventional programmed script. A limitation of this method is that the added rectangles may not always be laid out parallel with the current flow direction, particularly when the metal wire turns a corner and the direction of current flow changes. Consequently, manual intervention and modification is involved to correct the orientation of added rectangular slots.
There is a need, then, for an automated method of laying out cutting forms correctly oriented along the current flow direction of the metal wire.